Is there documentation that
explains how to interface to the sync pin on the Camera Link board?

There is a header connection point on the Camera Link
expansion board which provides access to the XP13 pin (External Sync)
(pin 21 Hirose). The connection point is through-hole #3 on the PCB (no
connector). Hole #1 is the square hole. This is a direct connection to
the FPGA, so the input levels should be 0V to 3.3V.